1. Field of the Invention
The present invention relates to an improved process for producing a SIALON sintered product which has less content of X phase.
2. Description of the Prior Arts
SIALON has the formula EQU Si.sub.6-z Al.sub.z O.sub.z N.sub.8-z
wherein z is 0 to 4.2 which has silicon nitride (Si.sub.3 N.sub.4) component and a solid solution component of Al.sub.2 O.sub.3.AlN spinel. SIALON is excellent as a heat resistant material as well as silicon nitride and silicon carbide.
Heretofore, the sintering of SIALON has been carried out by a hot pressing method. In accordance with the hot pressing method, a product having high density can be advantageously obtained whereas a product having complicated configuration can not be produced and a cost for the production is high to be disadvantageous.
Thus, the atmospheric pressure sintering method in which the powdery starting materials are molded and a molded product is sintered in nitrogen gas under the atmospheric pressure has been employed as the industrial process in view of an economical production and a mass production.
However, a thermal decomposition (pyrolysis) of SIALON is caused in the sintering step to prevent a contraction of the molded product and to vary the composition of the sintered product from that of the starting materials to be disadvantageous in the atmospheric pressure sintering method.
A method of preventing the change in composition materials during the sintering step has been proposed to overcome said disadvantages. For example, the following methods have been proposed.
(1) A method of covering the molded product with powdery Si.sub.3 N.sub.4. (2) A method of covering the molded product with a powder of the starting materials.
However, thermal decomposition in the molded product could not be prevented by these known methods and the sintered product having high density could not be obtained.
In the conventional sintering method, the starting materials give the formula having higher oxygen content comparing with the formula of EQU Si.sub.6-z Al.sub.z O.sub.z N.sub.8-z
have been used so as to result an easily sintering step.
However, in accordance with said method, the X phase (it is considered as Si.sub.9 Al.sub.7 O.sub.21 N.sub.9) which has higher oxygen content, has been included in the sintered product at relatively high ratio beside SIALON phase. The presence of the X phase causes lower melting point and inferior characteristics at high temperature, to be disadvantageous.